EP0771135B1 - Inductive Litz wire winding used in an induction cooking apparatus - Google Patents

Description

The present invention relates to cooking appliances by induction.

Induction cooking involves placing the substance to be cooked in a ferromagnetic vessel heated by a current flow in its walls caused by a high electromagnetic field frequency of the order of 20 to 50 kHz. The high electromagnetic field frequency is generated by an inductor coil which is arranged under the container and that is excited by a high alternating electric current frequency provided by an inverter.

The inductor winding usually consists of a coil flat made by winding, spirally joined, a strand of son stranded conductors individually insulated by an insulating layer.

Without a Litz-type stranding (periodic permutation of the strands in the strand), the proximity effect makes the high frequency current tend to migrate to the periphery of the strand, the outer conductors forming a screen around the internal conductors in which he hardly current. This results in improper use of the central strands of the strand and consequently a relatively high loss rate oversize the strand.

In the present art, conductive strands are isolated individually by a varnish to avoid a possibility of passage of the high frequency current between strands. The insulation varnish individually wire of the strand also ensures the electrical insulation between contiguous turns of the inductor winding. However, it has the disadvantage of increasing by significantly the cost of manufacturing the winding, this, all the more more than it must withstand temperatures of the order of 200 ° continuously, which can be reached by winding either because of its warming up because of a warm-up caused by the proximity of the cooking vessel.

To facilitate maintenance and cooling of the winding inductor, it is also known, in particular by the French patent application registered under No. 94 13653, to arrange the inductor winding in a hollow housing, spiral-shaped, dug on the upper face of a electrically insulating and thermally conductive support. In this case, turns of the inductor winding are no longer joined and are separated from each other by the walls of the housing. The insulating varnish of the strands strand then insures no electrical insulation between turns but only electrical insulation between adjacent strands at each turn.

The document FR 2 417 913 also describes an inductor winding having turns separated from each other by the walls of a housing made on a carrier of the inductor winding.

The present invention aims to reduce the cost of manufacture of a stranded multi-strand inductor winding of the Litz type for Induction cooking taking advantage of the electrical insulation between turns brought by the use of an electrically insulating coil carrier provided on its upper surface of a recessed, spiral-shaped impression serving as a housing for winding and having a partition wall between each turn of the winding.

It relates to a stranded multi-stranded inductor winding of the type Litz for Induction Cooker to have its spirals placed individually in a recessed spiral-shaped housing that is worn the upper face of an electrically insulating winding support and which has a partition wall between each turn of the winding. This stranded multi-strand inductor winding of Litz type is remarkable in that its stranded strands are made of bare conductive material.

Advantageously, the stranded multi-stranded inductor winding of Litz type has strands made of bare copper wire. He then has a level of high frequency losses about 15% higher than he would have if his strands were coated with an insulating varnish which is very reasonable.

Advantageously, the stranded multi-stranded inductor winding of Litz type with strands in bare aluminum wire. The bare aluminum overlaps immediately in contact with the air an electrically insulating oxide layer which makes it possible to limit the electrical contacts between strands so that the level of high frequency losses is very close to that of a multi-strand winding at strand type Litz, strand coated with an insulating varnish.

Other features and advantages of the invention will emerge from the following description of an embodiment given by way of example.
This description will be made with reference to the drawing whose single figure shows schematically, in section along a vertical plane, the possible structure of an induction cooking furnace equipped with a multi-stranded inductor winding according to the invention.

The induction cooking stove shown in the figure comprises an inductor winding 1 formed by a multi-strand strand of wires spiral-wound electrical conductors whose different turns are indexed by 10.

This inductive winding 1 is placed in a hollow recess in the form of a spiral, hollowed out in the upper face of a support of 2 electrically insulating coil. As can be seen in the figure, each turn 10 of the induction coil is placed in the cavity and is electrically insulated from its neighbor by the wall 11 of the imprint which realizes a partitioning. The footprint has a cross section slightly smaller than that of the multi-stranded strand of the in order to allow the fixing of the latter by jamming, by playing on the deformation of its cross-section which is oval. This avoids having to glue the turns of the inductor winding by means of a resin thermoadhérente.

The inductor winding 1 carried by its support 2 is placed under a plate 3, for example glass-ceramic, intended to receive the container to be heated. 4. It is preferably separated from the plate 3 by a layer 3a of thermal insulation (ceramic paper or glass wool) so to protect it from heating by the containers.

The support 2 of the induction coil 1 comprises known, a magnetic material 21 disposed for example from below, whose role is to focus the magnetic field generated by the winding inductor to the container to be heated.

The entire inductor coil 1 and its support 2 is placed on a plate 5 of non-magnetic material, for example aluminum, to enveloping edges, whose role is to minimize magnetic fields interference of the inductor winding 1, in particular with a view to protecting the generator of high frequency current placed underneath which feeds the inductor winding.

In high frequency (20-50 kHz) currents tend to flow around the periphery of the conductors. This phenomenon is called the film effect. With a round section conductor, 87% of the power is dissipated in the skin thickness p which is given by the formula: p = 1 μ 0 .μ r .σ.π. f where: μ ₀ = 4.π.10 ^-7 , μ _r = 1, σ = 58 Sm / mm ² and f is the frequency of the current.

By way of example, for round section copper at 25 kHz, it does not it is useless to exceed a diameter of 0.4 mm because, beyond, copper is very little used for the passage of currents. One round driver 0.4 mm is, however, clearly insufficient for the passage of currents excitation of a high power induction plate (3 kW). We can then play on the shape of the driver's section (flat) or put in works several drivers in parallel. In the latter case, we are confronted with a second effect called proximity effect. The distribution of currents in a driver is indeed disturbed by the passage of currents in a neighbor driver. In the case of several drivers meeting in beam, the outer conductors form a screen around the drivers in which there is almost no power. To fight against this phenomenon, it is necessary to adopt special stranding known as Litz stranding. Indeed, with a stranding classic, there is a central strand on which one comes to place layers concentric conductors and the screen effect is very pronounced. At contrary, with a Litz-type stranding, each elementary strand periodically place in the center of the strand and on its periphery. In addition, the fact optimize the current distribution gives the strands an impedance (L, R) substantially equivalent so that they are approximately equipotential at within the strand.

The inductor winding 1 is therefore formed of a multi-strand strand of wires conductors assembled in the manner of Litz wires so that each conductive elementary strand successively occupies all positions in the strand (in the center and the periphery ...).

As the electrical insulation between turns of the inductor winding is already provided by the partitioning 11 housing each of them in the winding support 2 there are only differences between strands residual potential at a turn that are of the order of some volts with a Litz-type stranding. These small differences in potential can cope with the natural resistance of contact between bare copper wire with round section because the contacts between strands are very punctual and therefore resistive, especially if the Litz-type stranding step is short, for example of the order of 35 mm for a strand of 20 strands conductors 0.4 mm in diameter.

Also, it is proposed to make the multi-strand winding strand inductor using elementary strands made of bare copper wire without varnish electrically insulating protection assembled in bundle according to a stranding Litz type. This results in a very significant reduction in the cost of inductor winding at the cost of an increase in the level of high frequency losses of about 15% which is very reasonable.

It would not be possible to use elementary strands bare copper without a Litz type stranding because then the impedances of the different strands in parallel to each other are strongly different, with, as a consequence, potential differences between strands well higher and a much higher level of high frequency losses in the case of strands electrically in contact.

It is also possible to use elementary strands made of aluminum wires bare without electrically insulating protective varnish. Aluminum presents on the copper the advantage of covering up immediately on contact with the air of an electrically insulating oxide layer leading to a resistance high contact between strands. The level of high frequency losses is then approaches that of a stranded multi-wire inductor winding of Litz, with copper strands isolated by a varnish. The advantage of using aluminum instead of copper also stems from its lower cost.

The removal of the electrically insulating varnish ensures, in addition a decrease in the cost of the inductor winding a better contact between strands and with the winding support which facilitates the winding cooling.

Claims (6)

1. Multi-filament inductive winding (1) with Litz stranding for an induction hob intended to have its turns (10) placed individually in a hollow housing in the form of a spiral which is carried by the upper face of an electrically insulating winding support (2) and which has a separating partition (11) between each turn (10), characterised in that said multifilament inductive winding consists of stranded filaments made from a bare conductive material.
2. Winding according to Claim 1, characterised in that its filaments are made from bare copper wires with no electrically insulating protective varnish.
3. Winding according to Claim 1, characterised in that its filaments are made from bare aluminium wires with no electrically insulating protective varnish.
4. Winding according to Claim 1, characterised in that the pitch of its Litz stranding is approximately 35 mm.
5. Winding according to Claim 4, characterised in that the conductive filaments of its strand have a diameter of approximately 0.4 mm.
6. Winding according to Claim 5, characterised in that it is formed from a strand of about twenty conductive filaments.

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